Several techniques are available for removing fine solid and liquid particulates from liquid hydrocarbon streams and various selection schemes have been investigated. Such separation depends primarily on particle surface characteristics and size distribution, specific gravity, electric and magnetic properties, concentration, liquid viscosity, liquid loss and cost. However, present technology is deficient in selective separation of moderate to high concentrations of very fine particles of mixed surface characteristics from hydrocarbon liquids with viscosities greater than 10 cp.
One method of interest is electrostatic coalescence. In comparison with other conventional separation technologies, such as filtration, sedimentation, hydrocycloning and centrifugation, electrostatic coalescence offers a low cost and simple operation. Electrostatic coalescence is used in the petroleum industry for dewatering and desalting of crude oil and for polishing distillates and highly refined products. In this method, emulsified water is used as a medium for absorbing the solids or other impurities to be removed. The electrostatic field causes the water to coalesce and the water and impurities are then separated by sedimentation. Present daily capacity of electrostatic desalters is over 8,000,000 barrels per day (bpd), with up to 4,000,000 bpd capacity for polishing operations.
The success of electrostatic coalescence in removing water and dissolved impurities from crude oil and petroleum products is highly sensitive to the choice of surfactants used to promote demulsification. The method is limited in its ability to remove all water and associated particulates from hydrocarbon liquids. Further, highly viscous crudes produced by steam injection contain significant amounts of emulsified water proving to be difficult to separate by conventional electrostatic methods. Additionally, the electrostatic method has not been successfully applied to breaking emulsions when the emulsions are stabilized by the presence of micron sized particulate matter. If the electrostatic method were applicable to handling emulsions containing high concentrations of solids, then the method could be extended to other applications of significance such as the recovery of hydrotreating and desulfurization catalysts such as zeolites or iron oxides from liquid hydrocarbon streams and to the processing of difficult to separate solids-stabilized emulsions which cause environmental problems in petroleum production and refining. In the synthetic fuels industry, the dispersion of water and fine mineral particles in oil necessitates the use of extensive oil-cleaning methods. Electrostatic coalesance has been investigated for dedusting of synthetic crudes such as coal liquefaction and shale retorting products. See L. A. Kaye and R. J. Fiocco, "Fine Particles Separations From NonAqueous Liquids," Separation Science and Technology, 19 (11 and 12), 794 (1984-85). The application of electrical coalescence to removal of dispersed water from coal-derived liquids has been discussed and the use of electrostatic coalescence methods for resolving emulsions made in some clean-up operations has been proposed.
An example of a significant problem of particulate separations occurs in the tar sands industry where very fine mineral particles must be removed from bitumen and other hydrophobic solids both in the recovery process and in the waste treatment stream. Electrostatic methods could potentially be used in these applications if efficient methods for selective separation of hydrophobic bitumen from hydrophilic mineral matter were available. Yet another example of a significant problem that might be addressed by electrostatic coalescense is that of separation of hydrophilic mineral matter from hydrophobic kerogen containing shale. Such beneficiation before retorting would improve the efficiency of conversion and greatly reduce the problems associated with removal of fine particulates from the shale oil.
Until now, it has been believed that electrostatic coalescence was associated with at least one of three types of behavior:
1. Chains of water droplets are formed, sometimes referred to as `Type I behavior.` See S. E. Taylor, "Investigations into the Electrical and Coalescence Behavior of Water-in-Crude Oil Emulsions in High Voltage Gradients," Colloids and Surfaces, 29 (1988) 29-51. The electrical attraction between the positive and negative poles induced by the applied electric field causes the droplets to coalesce.
2. Coalescence can also occur under more chaotic conditions. This behavior is sometimes referred to as `Type II behavior.` Under these conditions, each droplet exerts its own changing influence on the imposed field. Consequently, the position of each droplet and the electric field at every point fluctuate erratically. This causes the droplets to move about rapidly in random directions, which greatly increases the chances of coalescence.
3. Droplets are attracted to and collect on the electrodes forming larger and larger drops until eventually they fall by gravity.
The behavior of water-in-oil emulsions when subjected to high electric fields depends on the nature and quantity of the oil, water, and the presence of chemical additives in both water and oil phases, which subsequently affects the solubilities and the size and stability of water drops in the emulsion. Coalescence efficiency also depends on the method of emulsion preparation, the configuration of the electrodes and the operating conditions such as voltage, frequency, operation time, mechanical mixing, as well as the control of temperature and pH.
The handling of emulsions is greatly complicated by the presence of solid materials. Emulsions can be stabilized by the presence of solid or highly viscous matter at the interface of the two liquids. The removal of particles is affected by the surface characteristics of the solids and by the aqueous phase pH, high values of pH yielding better results. Further, fine sized particles have proven difficult to transfer into the water phase in the prior art. Emulsions containing micron sized particles are stable and have proven difficult to break, thus presenting a significant environmental problem in the petrochemical and alternative fuels industries.
While many problems of emulsion stability associated with the presence of fine particulates have been recognized prior to the work of this invention, there have been no successful attempts to separate solid particulates based on differences of surface characteristics when using electrostatic coalescence technology.